chore(deps): pin slackapi/slack-github-action action to 91efab1

Author: renovate[bot]

.github/actions/report-scheduled-failure/action.yml

@@ -10,7 +10,7 @@ runs:
   using: composite
   steps:
     - name: Report failure of a scheduled run
-      uses: slackapi/slack-github-action@v2.1.1
+      uses: slackapi/slack-github-action@91efab103c0de0a537f72a35f6b8cda0ee76bf0a # v2.1.1
       with:
         webhook: ${{ inputs.slack-webhook }}
         webhook-type: incoming-webhook